Apparatus for perforating thermoplastic sheet materials with an electric arc

ABSTRACT

Apparatus for perforating thermoplastic sheets by controlled high current density electric arcing through the sheet, wherein the high current density arc is established between electrically charged and grounded electrodes, both electrodes comprising discs having at least one pin mounted on the periphery thereof, the discs synchroneously rotating such that a grounding pin opposes a charged pin at the desired spacing of perforations to cause an electric arc to form said perforations.

1 1 Sept. 18, 1973 United States Patent Davies et a1.

[ APPARATUS FOR PERFORATING 3,167,641 Parmele et 219/384 THERMOPLASTICSHEET MATERIALS FOREIGN PATENTS OR APPLICATIONS WITH AN ELECTRIC ARC647,310 5/1939 Germany 219/384 Great Britain....... 4/1961Germany......... 0/1958 555,582 Canada........

[75] Inventors: Robert Dillwyn Davies, Wilmington,

De1.; Leslie Yarnall Weston, Jr., Avondale, Pa.

[73] Assignee: E. I. du Pont de Nemours and Primary ExammerVolodymyr Y.Mayewsky Company wllmmgton Attorney-Louis Del Vecchio Feb. 16, 1972 [22]Filed:

[57] ABSTRACT Apparatus for perforating thermoplastic sheets by con-[21] Appl. No.: 226,804

trolled high current density electric arcing through the sheet, whereinthe high current density are is established between electrically chargedand grounded electrodes, both electrodes comprising discs having atleast one pin mounted on the periphery thereof, the discs synchroneouslyrotating such that a grounding pin opposes a charged pin at the desiredspacing of perforations to cause an electric arc to form saidperforations.

References Cited UNITED STATES PATENTS 2,551,466 Salmon-Legaqneur etal....... 346/74 2,141,869 219/384 3 Claims, 6 Drawing Figures PATEN TEDSEP 1 8 I973 CURRENT APPARATUS FOR PERFORATING THERMOPLASTIC SHEETMATERIALS WITH AN ELECTRIC ARC BACKGROUND OF THE INVENTION Thisinvention relates to a process and apparatus for perforating athermoplastic film by an electric are discharged through the sheet.

Thermoplastic sheet materials have many widespread uses. One particularuse is as a packaging sheet material. When used to package some itemssuch as foodstuffs, e.g., fresh fruit or vegetables, it is desirable touse a packaging material that has holes in it to allow water to escape.In addition, it would be desirable to use a sheet material which is athermoplastic foam to provide cushioning for the item being packaged.Therefore, there is a need for means for perforating films includingfilms prepared from thermoplastic foams to be used for packagingfoodstuffs.

SUMMARY OF THE INVENTION Accordingly, this invention provides a processand apparatus for perforating thermoplastic sheet material. Theapparatus consists essentially of:

a. means for continuously advancing a thermoplastic sheet;

b. at least two electrodes positioned on opposite sides of the advancingsheet, wherein one electrode is electrically grounded and the otherelectrode is a pin electrode electrically charged and cyclically movingtoward the surface of the advancing sheet to initiate an arc and thenaway from the surface of the advancing sheet to extinguish the arc;

c. a high-voltage electrical power source to supply electrical power tothe cyclically moving electrode, wherein the electrical power isconnected to the electrode through a current-limiting choke to provide acontrolled high current density arc discharge at a relatively lowmaintenance voltage in the gap between the charged electrode and thegrounded electrode when the electrodes directly oppose each other;

d. means for controlling the size of the perforations and their spacingalong the length of the moving sheet within predetermined limits bycontrolling the current density and time duration of the are dischargeat preselected values to produce the desired hole size and preselectingthe cyclic frequency of the moving electrode in relation to the speed ofsheet advance to produce the desired hole spacing along the sheet.

In alternate embodiments, the apparatus additionally includes multiplecyclically moving electrodes incorporated and spaced laterally acrossthe sheet at intervals corresponding to the desired lateral spacing ofthe perforations with said electrodes supplied from a high voltagesource and opposed on the opposite side of the sheet by a groundedelectrode. In addition, each cyclically moving electrode supplied from ahigh voltage source can be a rotating disc having at least one pinmounted on the periphery of the disc and said disc electrode can beopposed by an electrically grounded disc electrode having pins mountedalong the periphery synchronously rotating with the high voltage discelectrodes providing a grounded pin to oppose each high voltage pin onopposite sides of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of apreferred embodiment of this invention showing a sheet material beingperforated by opposing electrodes. in addition, FIG. 1 shows a schematicof the electrical circuit used to operate the apparatus of the presentinvention.

FIGS. 2, 3 and 4 show a sequence of drawings illustrating an electricare being developed and extinguished.

FIG. 5 shows an alternate embodiment of the present invention whereinthe charged and grounded electrodes are both pin electrodessynchronously rotating to establish and extinguish an electrical arc.

FIG. 6 is a graph of Voltage v. Current as it applies to an electricalcircuit such as that used in the present invention wherein the arc ismaintained at a high current density but relatively low voltage acrossthe gap between the two electrodes.

DETAILS OF THE INVENTION This invention will be described in relation tothe drawings.

FIG. 1 is a schematic representation of an embodiment of the presentinvention. Sheet material 10 to be perforated is made to move in thedirection indicated by arrow 11 by conventional means not shown in thedrawings. The sheet material is made to pass over stationary barelectrode 12 which for safety reasons is grounded at 13. Optionally, barelectrode 12 can be cooled by pumping coolant through input port 14through the bar electrode 12 and out exit port 15. In addition, barelectrode 12 can be rotated by conventional mechanical means such aspulley system 16 driven by motor 17. Cooling is used to dissipate the10- calized heat generated at each arc location. Rotation of barelectrode 12 can be used to change that portion of the electrode surfaceexposed to the arc in order to reduce the surface erosion of the barelectrode.

Movable electrodes in the form of rotating discs 18 are mounted oninsulated shaft 19. The rotating disc electrodes 18 each carry at leastone pin electrode 20 on their periphery. Each disc is fastened toinsulated shaft 19 and made to rotate by conventional means such aspulley system 25 driven by motor 26.

The term pin electrode38 is a term of convenience and is meant toinclude an electrode that does address the sheet as a sharp pin, a bluntpin or a rod-like memher having a convexed end, i.e., such as ahemispherical shape, that addresses the sheet.

Electrical power is supplied to rotating discs 18 from a high voltagetransformer 21 wherein the secondary portion of the transformer has acapacitor 22 connected across it to provide power factor correction forthe inductive loading of each of the electrode circuits. Electricalpower leaves the transformer, passes through current-limiting chokes 23to disc electrodes 18. These chokes are required to balance and limitthe load to each electrode, i.e., divide the current evenly, because,without them, the first electrode to establish an arc would shortcircuit the transformer and reduce its voltage to a point where no otherelectrode would are. In situations where the desired hole spacingbetween electrodes in the lateral direction is so close that crossoverarcing from one disc to the other would take place, it is necessary toplace insulating shields (not shown) between the rotating discs 18 toprevent crossover arcing.

Electrical connection is made to each disc through a slip ring 24.

An expedient slip ring for each disc can be made from a ball bearing. Toconserve lateral space and permit close positioning of the discs, thebearing can be recessed in the disc with the outer bearing race pressedto fit. The inner bearing race is provided with a bore sufficientlylarger in diameter than shaft 19 so that a sleeve can be pressed intothis bore and also provide a clearance for shaft 19. This sleeve is inturn attached to a strap which acts as the electrical connector to choke23. This strap is fastened to the machine frame through suitableelectrical insulating means. Thus, the bearing will provide more thanadequate support for the shaft while acting as a current-carrying slipring for the electrodes.

An alternate arrangement which eliminates the need for individual sliprings in each disc includes a series of stationary pin electrodesreplacing the grounded bar electrode 12. Each of these pin electrodeswould be fed from the secondary side of transformer 21 through thecurrent-limiting chokes 23. Discs 18 carrying pin electrodes would thenbe carried on a conducting shaft instead of an insulating shaft and anelectrical return connection would be made through the shaft bearingsand machine frame to the power supply.

The electrical system described is designed to accommodate the use of aconventional, existing, altematingcurrent power source. Otherarrangements such as the use of individual transformers for eachelectrode disc can also be used. The electrical system required furtherdepends on whether DC or AC voltage is used and the frequency of the ACvoltage. AC voltage generally is preferred since the use of DC voltagerequires additional, generally more complex considerations for the highvoltage-high current supply and for the mode of current limitation.

In operation, power is supplied to the rotating discs and each time pinelectrode 20 opposes bar electrode 12, arcing occurs between the pinelectrode and the bar electrode perforating the sheet separating the twoelectrodes. The high current arcs so produced are difficult toextinguish. However, further rotation of the disc extinguishes the areas pin 20 moves away from bar electrode 12. Further rotation of the discwill again bring the pin electrode in opposing relationship to groundedbar electrode 12 while the sheet to be perforated is advancing and whenarcing occurs a second time a second hole is perforated in the sheet.Thus, as the sheet advances and the discs rotate, arcing occurs eachtime the pin electrode opposes stationary bar electrode and an array ofperforations is produced.

FIGS. 2, 3 and 4 show more clearly the sequence of action which occursas a disc carrying two pin electrodes is used to perforate sheetmaterial.

In FIG. 2, disc 27 carrying pin electrodes 28 and 29 is positioned sothat pin 28 opposes a grounded bar electrode 30. Power is supplied bymeans not shown and an are 31 is established between the two electrodesperforating the intermediate sheet 32. Referring to FIG. 3, as disc 27rotates, are 31 tends to elongate and follows pin 28 until pin 28 hasmoved a sufiicient distance away from electrode 30, as shown in FIG. 4,to extinguish itself, leaving intermediate sheet 32 with a perforation33 at the point where arcing passed through the sheet material. Thissequence will then repeat itself when pin electrode 29 now opposesgrounded bar electrode 30. It can be well appreciated that one or moreelectrodes can be located on the periphery of a disc electrode and, inaddition, the grounded electrode can also be a rotating disc having pinelectrodes mounted on the periphery sequenced to rotate and oppose thepin electrodes on the power-fed disc. Such an embodiment is shown inFIG. 5 wherein power-fed disc 34 is equipped with three pin electrodes35, 36 and 37 each opposed and synchronously rotating with pinelectrodes 38, 39 and 40 on grounded disc electrode 41.

While it is shown that the rotation of the disc is in the same directionthat the sheet material is made to travel, it has also been found thatrotation of the disc can be in a direction opposite to the travel of thesheet material. However, smaller holes are produced.

The relationship between the rotational speed of the power discs (W) inrpm and sheet speed (V) in feet per minute for a longitudinal holespacing of (S) in feet can be expressed in terms of the number of pinelectrodes (N) as follows:

- W= V/NS Thus, for a web velocity of 900 feet per minute a desired holespacing of 1 inch (1/12 foot) using a disc having two pin electrodes 180apart W= 900/(2Xl/12) 5400 rpm Note that the diameter of thedisc and thelength of the pin electrodes are not included in this relationship.These dimensions are governed by the voltage level required forperforation. Typically, for a 10 KV, 10 KHz voltage used to perforatefoam sheeting, an arc of about 7 inches develops before the arcextinguishes itself. Thus, if a disc radius of 2.25 inches and a pinlength of 2.25 inches is used, providing a total electrode radius of 4.5inches, the disc will rotate about for an arc path length of about 7.8inches without counting the added path length the arc must travel topass through the hole which has also moved in relationship to the arcingpin electrode. Therefore, extinction of the arc is assured before thesecond pin electrode on the disc opposes the grounded bar electrode forthe start of a new perforating sequence.

A system as shown in FIG. 1 has been assembled and used to perforate amicrofoam sheet (produced according to U. S. Pat. application Ser. No.797,312, filed Dec. 27, 1968, and now issued US. Pat. No. 3,637,458).Holes 3/16 inch in diameter spaced 2 inches apart across the sheet and 1inch apart in the direction of sheet advance were produced in saidmicrofoam sheeting having a thickness range of 60-90 mils. Four pinwheel electrodes with a total electrode radius of 2.80 inches and a discradius of 1.50 inches (pin length 1.30 inches) were used which containedtwo pin electrodes spaced apart on the disc periphery.

Discs 18 were spaced 2 inches apart on shaft 19 and were rotated at3,600 rpm to produce holes spaced 1 inch apart in the sheet moving at600 fpm. The pin electrodes, when in closest proximity to the foamsheeting surface, operated with an air gap of essentially 0.020 inch.Chokes 23 were typically 0.2 Henry with a DC resistance of less than 20ohms. Power factor correcting capacitors typically 0.001p.f wereconnected across each choke in place of capacitor 22. A capacitor 22with a value typically 0.004p.f would provide equivalent power factorcorrection; however, the use of individual capacitors at each chokepermits correction for variations in individual chokes and is apreferred arrangement. Transformer 21 was a well-regulated 7.5 KVA, KHzhigh-voltage transformer providing a gap voltage typically 10 KV rmsbefore breakdown. Gap voltage at breakdown was typically about 300 voltswith an initial current of l ampere.

An alternate arrangement, which would be generally higher in cost, wouldemploy an individual transformer to feed each electrode disc tocompletely avoid any electrode disc interaction. Transformers in thiscase should be a power-limiting type such that the secondary impedanceprovides the current-limiting function of chokes 23. A 220-V input,lS-KV output, 1.5-KVA transformer of this fonn was used successfully.The DC resistance of the secondary was 365 ohms. Power factor correctionwas provided by placing a capacitor of about luf across the primary ofthe transformer which had an inductance of about 0.25 Henry. In thiscase, as well as in those described supra, capacitance values are chosento provide a power factor approaching unity but avoiding resonance andits attendant instability during any part of the operating cycle.

It should be understood that other current-limiting devices, e.g.,capacitors, could be equally as effective as the chokes described.Chokes are preferred for the high current levels employed in practicingthis invention since they are generally less costly than capacitors inthis current-handling region. Chokes, additionally, are easily (and atlow cost) fabricated to meet certain nonlinear impedance-currentcharacteristics which can be useful for additional current control inthese perforation applications.

One of the important features of this invention is the fact thatcontrolled arcing can be used to systematically perforate athermoplastic sheet material by providing a high current density with arelatively low maintenance voltage between two electrodes placed onopposite sides of the thermoplastic sheet. The electrical aspectssignificant to the description of this important feature will bedescribed in relation to the accompanying drawings, particularly FIG. 6,showing a typical plot of voltage vs. current as it applies to theelectrical circuit used in the present invention and shown in FIG. 1.

Referring to FIG. 6, as voltage is applied and increased, the voltageacross the discharge gap increases as shown graphically in FIG. 6 from Oto A, at which point a value of gap voltage, V known as the sparkingpotential, is reached. With a further increase in voltage, the voltagein the gap remains essentially constant at this sparking potential levelof V However, the current flow through the current-limiting impedance,i.e., choke, and discharge gap increases as much as eight orders ofmagnitude, from essentially picoampere to microampere levels, as showngraphically from A to B.

At the higher current level near point B, space charge effects will bepredominant in the discharge gap with the result that the impedance ofthe gap will be significantly reduced so that a much lower gap voltageV, is required to maintain a slightly increased current, as showngraphically from B to C. When this impedance change occurs in the gap,the voltage drop across current limiters increases as the currentflowing through the series circuit of the current limiters and dischargegap increases from B to C.

As the current is further increased by further increasing the appliedvoltage, the gap voltage remains essentially constant at a value of V,,,and the discharge (a glow discharge) spreads over the surface of theelectrodes, and as it spreads, the current density remains essentiallyconstant while current flow in the gap increases as shown from C to D.When the whole electrode surface is covered by this glow discharge, thegap voltage must increase to increase the current (and hence the currentdensity), as shown from D to 15. At this high current level, the heatingeffect (ohmic or I R heating) of the high current density both on theelectrodes and on the surrounding gaseous medium in the gap producesadditional electrons and ions. This again significantly reduces the gapimpedance such that a high current is maintained at a substantiallyreduced gap voltage V,,. The gap breakdown is quite pronounced as shownfrom E to F.

The variable nature of the impedance of the gap as the applied voltageis increased, makes it evident that a series current-limiting impedanceis essential if an applied voltage is anticipated which will exceed avalue of V Without such a current limiter, there would be no control ofthe discharge current in the gap. From further consideration of FIG. 6,it is evident that operation of a discharge gap at a point to the leftof point C will usually be at an energy level too low to be useful forperforating thermoplastic sheet materials. In the region from C to E,the energy level is in a range found useful for perforating according tothe prior art (see U. S. Pat. No. 3,385,951, Bancroft et al., Col. 3,Line 27, to C01. 4, Line 4). The region from D to E has been found to beparticularly useful, since the slope of the voltagecurrent curve makesthe state of the art control techniques easily applicable to the controlof the energy level in each discharge for the length of time availablein the process for perforating. The region beyond E has been avoided dueto the hitherto lack of control of the discharge and the difficulties ofarc extinction attendant with the low voltage (V,, required to maintainthe high current discharge in this region. For the puspose ofclarification of terminology, V and V are defined as the glow dischargeand are discharge maintenance voltages, respectively. V, is alsosynonymous with the maintenance voltage for what is commonly called thespark which is produced at pressures greater than mm. Hg. The dischargein the region C D E is often erroneously called an arc, since arcs occurin the region E F The operating regions depicted in FIG. 6 are dependenton the applied voltage and the circuit impedance. As ari example, for agap of 0.100 inch the gap voltages described supra would typically be:

at atmospheric pressure.

Normally, the applied voltage has a fixed value, predetermined to besufficient to cause initial breakdown of the dielectric material in thedischarge gap (the thermoplastic sheet and air gap). The impedance valueof the current limiters is chosen with a predetermined value such that,following initial breakdown, the desired current and correspondingvoltage in the discharge gap will be maintained. Once breakdown occursin the gap, the operating point on the curve is reached almostinstantaneously (limited only by the time constants of the circuitelements, which are generally in the microsecond range). Thus, if thesystem is designed to operate at point D of FIG. 6, with the applicationof voltage, the voltage profile in the gap will follow that indicated bythe curve as shown from f 1 B C D and operation at point D will occuressentially instantaneously.

Perforation of sheet material to provide invisible holes forbreathability can be achieved by operating in region B to D. To produceholes with a nominal 0.030 inch diameter as described in U. S. Pat. No.3,385,951, supra, is achieved by operating in the D to E region with aminimum discharge time of 50 milliseconds (Col. 3, Line 60). Productionof holes 1/8 inch to 3/8 inch diameter operating in this region wouldrequire an impractically long time for normal sheet processing speeds,if possible at all.

It was found that operation in the arc discharge region described bythat portion of the graph of FIG. 6 from point F and beyond was requiredto produce perforations in the size range of 16 inch diameter andlarger. The high current density arc releases its high energy in about10 seconds (1 microsecond) and essentially blasts the material from thesheet to form the hole. In the system described, the high currentdensity arc exists typically for 8 milliseconds; the major portion ofthis time period is required to extinguish the are.

A typical voltage waveform can be viewed by connecting an oscilloscopeacross a discharge gap. With the applied voltage supplied from a 10 KV,10 KHz source, voltage at breakdown would drop from 10 KV to typically300 V rms when the electrodes are brought together. As the electrodesseparate, the gap voltage increases until extinction after about 8milliseconds. The voltage remains at 10 KV for about 4 millisecondsafter which arcing occurs again between the next electrode pair. Currentflowing in the gap is the inverse of the above, being a maximum atbreakdown (typically greater than 0.5 amps ms) and decaying to a lowvalue (less than 10 amps) when discharge ceases.

This invention is applicable to perforating many types of thermoplasticsheets including polyolefin sheets, e.g., polyethylene andpolypropylene; polyester films, e. g., polyethylene terephthalate;polyvinyl sheets, e.g., polyvinyl chloride; polyamides, e.g., nylon.This invention is particularly useful in perforating thermoplastic foamsheets such as the microcellular polyolefin foams of U. S. Pat.application Ser. No. 797,312, filed Dec. 27, 1968, and now issued U.S.Pat. No. 3,637,458,

because it perforates the foam leaving neat perforations withoutdestroying immediately adjacent foam cells.

We claim:

1. An apparatus for forming perforations in a thermoplastic sheetcomprising:

a. means for continuously advancing a thermoplastic sheet;

b. positioned above one side of the advancing sheet, multipleelectrically charged discs having at least one pin mounted on theperiphery of each disc, said discs spaced laterally across the sheet atintervals corresponding to the desired lateral spacing of theperforations and rotating to cyclically move said pins toward thesurface of the advancing sheet to initiate an arc and then away from thesurface of the advancing sheet to extinguish the arc;

. positioned above the opposite side of the advancing sheet and opposingeach electrically charged disc, multiple grounded discs having at leastone grounding pin mounted on the periphery of each disc, said groundeddiscs synchronously rotating with the electrically charged discelectrodes providing a grounding pin to oppose the electrically chargedpins;

d. a high-voltage electrical power source to supply electrical power tothe rotating discs and the cyclically moving electrically charged pins,wherein the electrical power is connected to the discs through acurrent-limiting choke to provide a controlled high current density aredischarge at a relatively low maintenance voltage in the gap between acharged disc pin and a grounded disc grounding pin when the charged discpin and grounding pin directly oppose each other; and

means for controlling the size of the perforations and their spacingalong the length of the moving sheet within predetermined limits bycontrolling the current density and time duration of the are dischargeat preselected values to produce the desired hole size and preselectingthe cyclic frequency of the moving electrode in relation to the speed ofsheet advance to produce the'desired hole spacing along the sheet. 2.The apparatus of claim 1 wherein the rotating disc electrode has twopins mounted 180 apart on the periphery of the electrode.

3. The apparatus of claim 1 wherein the rotating disc electrode hasthree pins mounted apart on the periphery of the electrode.

rw UNITED STATES PATENT OFFICE. CERTIFICATE OF CORRECTION patent3,260,153 D d September 18, 1973 ,r 'srorfvs) Robert Dillwvn Davies andLeslie Yarnall WestonkJr.

It is certified that error appears in the above-identified patent V andthat said Letters Patent are hereby corrected as shown below:

T v Column 1, line 64, of the discs with the elec- 'trioally groundeddisc electrodes should appear after "periphery r I Column Z, line 46;'pin electrode 38" should read "pin electrode Signed and sealed-[this1st day of January 197M.

(SEAL) Attest:

EDWARD MJLETCEEEJE. RENE D. TEGTMEYEE Attesting Officer ActingCommissioner of Patents I iatent No. ,260,153

213323? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dat dSeptember 18, 1.973

Robert Dillwyn Davies and Leslie Yarnall WestonLJr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

columnl, line 64,'-- of the discs with the elec- 'trically grounded discelectrodes should appear after "periphery.

Column 2, line 46, u in electrodeaguw read "pin electrode? Signed andsealed this 1st day of January 197M. I

(S I J C v Attest:

EDWARD M.FLETCHER,JR. 1 RENE D. TEGTMEYER Acting Commissioner of PatentsAttesting Officer 7

1. An apparatus for forming perforations in a thermoplastic sheetcomprising: a. means for continuously advancing a thermoplastic sheet;b. positioned above one side of the advancing sheet, multipleelectrically charged discs having at least one pin mounted on theperiphery of each disc, said discs spaced laterally across the sheet atintervals corresponding to the desired lateral spacing of theperforations and rotating to cyclically move said pins toward thesurface of the advancing sheet to initiate an arc and then away from thesurface of the advancing sheet to extinguish the arc; c. positionedabove the opposite side of the advancing sheet and opposing eachelectrically charged disc, multiple grounded discs having at least onegrounding pin mounted on the periphery of each disc, said grounded discssynchronously rotating with the electrically charged disc electrodesproviding a grounding pin to oppose the electrically charged pins; d. ahigh-voltage electrical power source to supply electrical power to therotating discs and the cyclically moving electrically charged pins,wherein the electrical power is connected to the discs through acurrent-limiting choke to provide a controlled high current density arcdischarge at a relatively low maintenance voltage in the gap between acharged disc pin and a grounded disc grounding pin when the charged discpin and grounding pin directly oppose each other; and e. means forcontrolling the size of the perforations and their spacing along thelength of the moving sheet within predetermined limits by controllingthe current density and time duration of the arc discharge atpreselected values to produce the desired hole size and preselecting thecyclic frequency of the moving electrode in relation to the speed ofsheet advance to produce the desired hole spacing along the sheet. 2.The apparatus of claim 1 wherein the rotating disc electrode has twopins mounted 180* apart on the periphery of the electrode.
 3. Theapparatus of claim 1 wherein the rotating disc electrode has three pinsmounted 120* apart on the periphery of the electrode.